Family history studies showed a familial aggregation of Alzheimer disease among first-degree relatives of Alzheimer's disease probands compared with controls. A case report shows that dementia in Down syndrome may occur without mental retardation. DNA repair studies show a G2 excision-repair deficiency in Down syndrome and sporadic and familial Alzheimer disease. APOE4 allele in Down syndrome increases the age-related incidence of dementia and shortens life expectancy.